This invention relates generally to casing hardware of the type used in cementing of oil or gas wells. More particularly, the invention covers casing structures, such as float collars and shoes, which have a concrete core therein. The design of this casing structure places the concrete core under a predominantly compressive force.
The preparation of an oil well borehole for recovery of oil or gas involves a step referred to as primary cementing. In a typical primary cementing operation, a cement-water slurry is pumped down the well borehole through steel casing to critical points located in the annulus around the casing. There are several reasons for cementing an oil well. For example, cementing prevents flow of connate water into possible productive zones behind the casing, and it protects the casing against corrosion from subsurface mineral waters. Cementing also minimizes the hazard of polluting, with oil and salt water, supplies of fresh drinking water and recreational water contained in rock strata adjacent to the well. Other reasons for cementing the well are to prevent blow-outs and fires caused by high pressure gas zones behind the casing, and to prevent the casing from collapsing as a result of high external pressures which can build up underground.
In some cementing operations, the casing hardware includes pieces referred to as float collars and float shoes. The float collar is attached near the end of the casing and below that is another piece of casing known as a shoe joint, which couples the collar to the float shoe. In both the float collar and the shoe is a check valve, which is held in place by a core, which consists of a solid, drillable material. As the casing is lowered into the borehole, prior to injection of cement into the casing, the check valves are in a "closed" position. This prevents the casing from filling with drilling mud and other fluids in the hole. The word "float" implies that the casing will not fill with fluids, unless it is filled from the surface, so that these structures have enough buoyancy to float, or partially float, in the fluid and thus reduce the weight of the casing considerably.
During displacement of the cement slurry into the borehole annulus, the check valves are in an "open" positon. Once the desired amount of cement has been pumped into the annulus, the pumping is stopped, and the valves move back to a closed position. At this point in the operation, the level of cement in the annulus is somewhere above the check valves. Since the cement is much heavier than the displacement fluid, the cement column is in an "unbalanced" condition, and the closed valves retain the cement in this condition until it sets up. The solid concrete core and the check valves inside the float collar and shoe are then drilled out to prepare the well for the next step in the recovery operation.
The float collars and shoes in use today, as well as differential fill, orifice, and guide equipment, have a casing structure with a solid, drillable core material inside the casing. The purpose of the core material is to support a valve, or to provide a solid, drillable material for various other functions. In the present casing hardware, particularly float collars and shoes, the usual core materials are concrete, aluminum, or phenolic resin compositions. The casing structures equipped with concrete cores have a structural weakness which makes them unsatisfactory for general downhole use. An example of such equipment is the conventional float collar illustrated in FIG. 1.
As shown in FIG. 1, the inside surface of the casing structure 10 of the float collar resembles a corrugated surface, that is, it has alternating ridges 11 and grooves 12. The purpose of the corrugated surface is to provide a means for anchoring the concrete core 13 to the casing. When the concrete core 13 hardens inside the casing structure 10, the casing exerts a force against the core in a direction which is normal to the sloping sides 14 of the ridges 11. The force which is applied to the concrete core, as indicated by the broken line arrows 15, is predominantly a shearing force.
As the float is lowered into the borehole, the ball 16 in the check valve settles into a seat at the top of the ball cage 17, so that the valve is then in its closed position. When the check valve is in closed position, there is a substantial amount of upward pressure against the ball and the top of the ball cage and against the bottom face of the concrete core. This pressure is exerted by the drilling mud and other fluids in the borehole while the casing is being floated into place. Additional pressure is also exerted against the concrete core and the ball and cage top after the valve closes to retain the cement column in its unbalanced condition, as described earlier. Fluids above the concrete core also exert a substantial amount of downward pressure against the top face of the core. In actual practice, the pressure differential from above the core is usually greater than from below.
The ability of the concrete core to resist these pressure forces is entirely dependent on its shear strength. When the pressure forces exceed the shear strength of the core 13, the core usually fractures along the "shear" lines 18. The usual result is that the top section of the core (above the fracture line) along with the ball 16, and the top of the ball cage, separates from the bottom section of the core (below the fracture line), and allows fluid to by-pass the check valve.
From past studies, it is known that cement and cement aggregates are much stronger when placed in conditions of compression than in conditions of shear. This principle is utilized in the present invention to provide a new design for casing structures which improve the ability of the concrete core to withstand pressures which substantially exceed the pressure limits of the casing structure cores now in use.